U.S. patent application number 15/100747 was filed with the patent office on 2016-10-13 for exhaust gas purification device and particulate filter.
This patent application is currently assigned to Cataler Corporation. The applicant listed for this patent is CATALER CORPORATION. Invention is credited to Tsuyoshi ITO, Tatsuya OHASHI, Ryota ONOE, Shingo SAKAGAMI.
Application Number | 20160296873 15/100747 |
Document ID | / |
Family ID | 53273432 |
Filed Date | 2016-10-13 |
United States Patent
Application |
20160296873 |
Kind Code |
A1 |
ONOE; Ryota ; et
al. |
October 13, 2016 |
EXHAUST GAS PURIFICATION DEVICE AND PARTICULATE FILTER
Abstract
Particulate filter provided in the exhaust gas purification
device includes: a wall-flow part having an inlet-side cell that is
open only at an end on an exhaust gas inflow side, outlet-side cell
adjacent to this inlet-side cell and is open only at an end on an
exhaust gas outflow side, a porous wall partitions the inlet-side
cell from the outlet-side cell; a straight-flow part having a
through cell that penetrates the filter in axial direction and is
open at the end on the exhaust gas inflow side as well as the end
on the exhaust gas outflow side. In a cross section of the filter
orthogonal to its axial direction, cross-sectional areas of the
inlet-side cell and outlet-side cell present in an outer peripheral
region of the cross section are larger than cross-sectional areas
of the inlet-side cells and the outlet-side cell in a central
region of the cross section.
Inventors: |
ONOE; Ryota; (Kakegawa-shi,
JP) ; SAKAGAMI; Shingo; (Kakegawa-shi, JP) ;
ITO; Tsuyoshi; (Kakegawa-shi, JP) ; OHASHI;
Tatsuya; (Kakegawa-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CATALER CORPORATION |
Kakegawa-shi, Shizuoka |
|
JP |
|
|
Assignee: |
Cataler Corporation
Kakegawa-shi, Shizuoka
JP
|
Family ID: |
53273432 |
Appl. No.: |
15/100747 |
Filed: |
December 1, 2014 |
PCT Filed: |
December 1, 2014 |
PCT NO: |
PCT/JP2014/081784 |
371 Date: |
June 1, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N 2330/38 20130101;
F01N 2330/34 20130101; F01N 3/0222 20130101; B01D 46/247 20130101;
F01N 2330/30 20130101 |
International
Class: |
B01D 46/24 20060101
B01D046/24; F01N 3/022 20060101 F01N003/022 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 2, 2013 |
JP |
2013-249169 |
Claims
1. An exhaust gas purification device disposed in an exhaust
passage of an internal combustion engine and provided with a
particulate filter capturing granular matter in an exhaust gas
discharged from the internal combustion engine, wherein the
particulate filter includes: a wall-flow part including an
inlet-side cell that is open only at an end on an exhaust gas
inflow side, an outlet-side cell that is adjacent to the inlet-side
cell and that is open only at an end on an exhaust gas outflow
side, and a porous wall that partitions the inlet-side cell from
the outlet-side cell; and a straight-flow part including a through
cell that penetrates the filter in an axial direction thereof and
that is open at the end on the exhaust gas inflow side as well as
the end on the exhaust gas outflow side, and wherein in a cross
section of the filter orthogonal to the axial direction, an outer
peripheral region and a center region of the cross section are
respectively provided with a plurality of inlet-side cells,
outlet-side cells, and through cells, cross-sectional areas of each
of the inlet-side cells and each of the outlet-side cells present
in the outer peripheral region of the cross section are larger than
cross-sectional areas of each of the inlet-side cells and each of
the outlet-side cells present in the central region of the cross
section, and a cross-sectional area of each of the through cells
present in the outer peripheral region of the cross section is
smaller than a cross-sectional area of each of the through cells
present in the central region of the cross section.
2. The exhaust gas purification device according to claim 1,
wherein a cross section of the filter orthogonal to the axial
direction is approximately circular, and when the radius of this
cross section is defined as R, the central region is defined as a
region from the center of the cross section to at least 1/2R of the
radius R, and the outer peripheral region is defined as a region
from the outer edge of the cross section to at least 1/5R of the
radius R.
3. The exhaust gas purification device according to claim claim 1,
configured such that when 100% is a total amount of exhaust gas
passing through the central region, the amount of exhaust gas
passing through the wall-flow part of the central region is 90% to
99%.
4. The exhaust gas purification device according to claim 1,
configured such that when 100% is a total amount of exhaust gas
passing through the outer peripheral region, the amount of exhaust
gas passing through the wall-flow part of the outer peripheral
region is 92% to 100%.
5. The exhaust gas purification device according to claim 1,
wherein the cross-sectional areas of the individual inlet-side
cells and the individual outlet-side cells present in the outer
peripheral region are approximately equal and are uniformly larger
than the cross-sectional areas of the inlet-side cells and the
outlet-side cells present in the central region.
6. The exhaust gas purification device according to claim 1,
wherein inlet-side cells and outlet-side cells are formed in the
filter that have a cross-sectional area that gradually increases
from the center of the aforementioned cross section toward the
outer edge.
7. The exhaust gas purification device according to claim 1,
wherein the wall-flow part has a plurality of the inlet-side cells
and a plurality of the outlet-side cells disposed in alternation in
a grid form, and the through cell is disposed along the diagonal
direction of the grid and between an inlet-side cell and an
inlet-side cell adjacent thereto and between an outlet-side cell
and an outlet-side cell adjacent thereto.
8. The exhaust gas purification device according to claim 1,
wherein a through cell cross section orthogonal to the axial
direction of the filter has a quadrilateral shape, and an
inlet-side cell cross section orthogonal to the axial direction of
the filter and an outlet-side cell cross section orthogonal to the
axial direction of the filter have an octagonal shape.
9. The exhaust gas purification device according to claim 1,
wherein the internal combustion engine is a gasoline engine.
10. A particulate filter that is provided in an exhaust gas
purification device according to claim 1.
11. A particulate filter disposed in an exhaust passage of an
internal combustion engine and capturing granular matter in an
exhaust gas discharged from the internal combustion engine, the
particulate filter comprising: a wall-flow part including an
inlet-side cell that is open only at an end on an exhaust gas
inflow side, an outlet-side cell that is adjacent to the inlet-side
cell and that is open only at an end on an exhaust gas outflow
side, and a porous wall that partitions the inlet-side cell from
the outlet-side cell; and a straight-flow part including a through
cell that penetrates the filter in an axial direction thereof and
that is open at the end on the exhaust gas inflow side as well as
the end on the exhaust gas outflow side, wherein in a cross section
of the filter orthogonal to the axial direction, an outer
peripheral region and a center region of the cross section are
respectively provided with a plurality of inlet-side cells,
outlet-side cells, and through cells, cross-sectional areas of each
of the inlet-side cells and each of the outlet-side cells present
in the outer peripheral region of the cross section are larger than
cross-sectional areas of each of the inlet-side cells and each of
the outlet-side cells present in the central region of the cross
section, and a cross-sectional area of each of the through cells
present in the outer peripheral region of the cross section is
smaller than a cross-sectional area of each of the through cells
present in the central region of the cross section.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purification
device that is disposed in an exhaust passage of an internal
combustion engine. More particularly, the present invention relates
to an exhaust gas purification device that is provided with a
particulate filter that captures granular matter in the exhaust gas
discharged from an internal combustion engine.
[0002] This international application claims priority based on
Japanese Patent Application No. 2013-249169 filed Dec. 2, 2013, and
the contents of this application are incorporated in their entirety
in the present Description by reference.
BACKGROUND ART
[0003] It is generally known that, inter alia, particulate matter
(PM), which has carbon as its main component, and ash, which is
composed of uncombusted components, are present in the exhaust gas
discharged from internal combustion engines and are a cause of air
pollution. Due to this, the regulations on particulate matter
emission levels, along with those on harmful components present in
exhaust gas, e.g., hydrocarbon (HC), carbon monoxide (CO), and
nitrogen oxides (NOx), are becoming more rigorous with each passing
year. Art has thus been proposed for capturing and thereby removing
this particulate matter from exhaust gases.
[0004] For example, a diesel particulate filter (DPF) for capturing
this particulate matter has been disposed in the exhaust passage of
diesel engines. In addition, gasoline engines discharge a certain
amount of particulate matter with their exhaust gas, although this
amount is smaller than for diesel engines, and as a consequence in
some instances a gasoline particulate filter (GPF) is also
installed in the exhaust passage of gasoline engines. Particulate
filters having a wall-flow structure are known here; these are
constructed from a large number of cells composed of a porous
substrate wherein the inlets and outlets of the large number of
cells are blocked in alternation. In a wall-flow particulate
filter, the exhaust gas that has entered through a cell inlet
passes through the porous cell walls provided as partitions and is
discharged towards and at a cell outlet. While the exhaust gas is
traversing the porous cell walls, the particulate matter is
captured and removed at the wall surface and in the pores in the
interior of the wall. Patent Literature 1 is an example of this
type of prior art.
CITATION LIST
Patent Literature
[0005] [Patent Literature 1] Japanese Patent Application Laid-open
No. 2003-210992
SUMMARY OF INVENTION
[0006] There is, however, a limit on the amount of particulate
matter that can be captured by the cell wall in the wall-flow
particulate filter described above, and when particulate matter
accumulates in the filter in an amount exceeding this, filter
clogging occurs and a large pressure loss then appears. This can
result in the appearance of adverse effects such as a deterioration
in the fuel consumption efficiency (fuel efficiency) and engine
problems. Due to this, with, for example, DPFs, once a prescribed
amount or more of particulate matter has accumulated in the filter,
filter regeneration is carried out by establishing a
high-temperature exhaust gas flow and burning off the particulate
matter. With GPFs, for example, filter regeneration is performed by
burning off the particulate matter during a fuel cut-off
interval.
[0007] However, when, for example, a satisfactory regeneration
treatment has not been performed due to control deficiencies, or
when an operating condition with a relatively low exhaust
temperature has persisted, e.g., during engine start up or idling,
an abnormal PM accumulation occurs and filter clogging then occurs
and this results in the problem of an increased pressure loss.
Increases in the pressure loss are desirably kept as small as
possible with no decrease in PM trapping efficiency.
[0008] The present invention was pursued considering these points,
and its primary object is to provide, for the instant particulate
filter and for an exhaust gas purification device equipped with
this particulate filter, a novel structure that can suppress
pressure loss increases while maintaining the PM trapping
efficiency.
[0009] The exhaust gas purification device provided by the present
invention is an exhaust gas purification device that is disposed in
an exhaust passage of an internal combustion engine and that is
provided with a particulate filter that captures granular matter in
an exhaust gas discharged from the internal combustion engine. This
particulate filter is provided with a wall-flow part that has an
inlet-side cell that is open only at an end on an exhaust gas
inflow side, an outlet-side cell that is adjacent to the inlet-side
cell and that is open only at an end on an exhaust gas outflow
side, and a porous wall that partitions the inlet-side cell from
the outlet-side cell; and is provided with a straight-flow part
containing a through cell that completely penetrates the filter
along the axial direction and that is open at both the end on the
exhaust gas inflow side and the end on the exhaust gas outflow
side. And, in a cross section of the filter orthogonal to the axial
direction, cross-sectional areas of an inlet-side cell and an
outlet-side cell present in an outer peripheral region of the cross
section are larger than cross-sectional areas of an inlet-side cell
and an outlet-side cell present in a central region of the cross
section.
[0010] Due to this construction, the exhaust gas preferentially
flows in the straight-flow part when the accumulation of
particulate matter (PM) has advanced in the wall-flow part into
which exhaust gas has been introduced, and because of this
increases in the pressure loss can be kept low for the particulate
filter as a whole. In addition, because the cross-sectional areas
of the inlet-side cell and outlet-side cell present in the outer
peripheral region of the filter are larger than those of the
inlet-side cell and outlet-side cell present in the central region,
even when clogging has occurred in the central region of the
filter, the PM can still be thoroughly captured in the outer
peripheral region with its large trapping capacity. Accordingly,
pressure loss increases can be restrained while maintaining the PM
trapping efficiency.
[0011] In a preferred aspect of the herein disclosed exhaust gas
purification device, a cross section of the filter orthogonal to
its axial direction is approximately circular, and, defining R as
the radius of this cross section, the aforementioned central region
is defined as the region from the center of the cross section to at
least 1/2R of the radius R, and the aforementioned outer peripheral
region is defined as the region from the outer edge of the cross
section to at least 1/5R of the radius R. The PM can be uniformly
captured in the filter as a whole by defining the central region
and the outer peripheral region in this manner. As a result,
pressure losses are reduced and excellent filter characteristics
are maintained.
[0012] A preferred aspect is configured such that the amount of
exhaust gas passing through the wall-flow part of the central
region is 90% to 99% where 100% is the total amount of exhaust gas
passing through the central region. Due to this construction, the
ratio for the central region of the filter between the amount of
exhaust gas passing through the wall-flow part and the amount of
exhaust gas passing through the straight-flow part assumes a
favorable balance, and as a result for the central region of the
filter the pressure loss increases caused by clogging can be
restrained while maintaining the PM trapping efficiency.
[0013] In addition, a preferred aspect is configured such that the
amount of exhaust gas passing through the wall-flow part of the
outer peripheral region is 92% to 100% where 100% is the total
amount of exhaust gas passing through the outer peripheral region.
Due to this construction, the ratio for the outer peripheral region
of the filter between the amount of exhaust gas passing through the
wall-flow part and the amount of exhaust gas passing through the
straight-flow part assumes a favorable balance, and as a result for
the outer peripheral region of the filter the pressure loss
increases caused by clogging can be restrained while maintaining
the PM trapping efficiency.
[0014] In a preferred aspect of the herein disclosed exhaust gas
purification device, the cross-sectional areas of the inlet-side
cells and outlet-side cells present in the outer peripheral region
are approximately equal and are uniformly larger than those of the
inlet-side cells and outlet-side cells present in the central
region. Due to this construction, reductions in the PM trapping
efficiency can be securely and reliably suppressed.
[0015] In a preferred aspect of the herein disclosed exhaust gas
purification device, inlet-side cells and outlet-side cells are
formed in the filter that have a cross-sectional area that
gradually increases from the center of the aforementioned cross
section toward the outer edge. The use of such an exhaust gas
purification device makes it possible to bring about a more
sensitive tuning of the flow rate of the exhaust gas passing
through the inlet-side cells and outlet-side cells (wall-flow part)
in the filter moving from the center of the cross section to its
outer edge. This in turn makes possible a better improvement in the
PM trapping efficiency.
[0016] In a preferred aspect of the herein disclosed exhaust gas
purification device, in the cross section orthogonal to the axial
direction of the filter, the cross-sectional area of the through
cells present in the outer peripheral region of this cross section
is smaller than the cross-sectional area of the through cells
present in the central region of this cross section. This
construction makes it possible to simply and easily realize a
construction in which the cross-sectional areas of the inlet-side
cell and outlet-side cell in the outer peripheral region are larger
than the cross-sectional areas of the inlet-side cell and
outlet-side cell in the central region. This ensures suppression of
decrease in PM trapping efficiency.
[0017] In a preferred aspect of the herein disclosed exhaust gas
purification device, the wall-flow part has a plurality of the
inlet-side cells and a plurality of the outlet-side cells disposed
in alternation in a grid form. In addition, each of through cells
is disposed between an inlet-side cell and an inlet-side cell
adjacent thereto and between an outlet-side cell and an outlet-side
cell adjacent thereto, disposed along the diagonal direction of the
grid formed by the inlet-side cells and outlet-side cells. Due to
this construction, exhaust gas overflowing from the wall-flow part
rapidly flows into the through cells and as a consequence an even
better restraint on pressure loss increases can be exercised. That
is, the effects of the present invention can be exhibited at an
even higher level.
[0018] In a preferred aspect of the herein disclosed exhaust gas
purification device, the through cell cross section orthogonal to
the axial direction of the filter has a quadrilateral shape and the
inlet-side cell cross section orthogonal to the axial direction of
the filter and the outlet-side cell cross section orthogonal to the
axial direction of the filter have an octagonal shape. With each
cell being designed differently as such, an efficient cell
arrangement can be realized in the limited cell-occupied space of
the filter.
[0019] The present invention also provides a particulate filter
included in any exhaust gas purification device disclosed herein.
That is, this is a particulate filter disposed in an exhaust
passage of an internal combustion engine and capturing granular
matter in the exhaust gas discharged from the internal combustion
engine. This particulate filter includes a wall-flow part including
an inlet-side cell that is open only at an end on an exhaust gas
inflow side, an outlet-side cell that is adjacent to the inlet-side
cell and that is open only at an end on an exhaust gas outflow
side, and a porous wall that partitions the inlet-side cell from
the outlet-side cell, and also includes a straight-flow part
containing a through cell that penetrates the filter in an axial
direction thereof and that is open at the end on the exhaust gas
inflow side as well as the end on the exhaust gas outflow side. In
addition, in the cross section orthogonal to the axial direction of
this filter, cross-sectional areas of the inlet-side cell and
outlet-side cell present in the outer peripheral region of the
cross section are larger than cross-sectional areas of the
inlet-side cell and outlet-side cell present in the central region
of the cross section. The use of this particulate filter makes
possible the realization of a high-performance exhaust gas
purification device that can restrain pressure loss increases while
maintaining its PM trapping efficiency.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a diagram that schematically shows an exhaust gas
purification device according to an embodiment;
[0021] FIG. 2 is a perspective diagram that schematically shows a
filter according to an embodiment;
[0022] FIG. 3 is a diagram that schematically shows the relevant
portion of the end face of a filter according to an embodiment;
[0023] FIG. 4 is a diagram that schematically shows the IV-IV cross
section of FIG. 3;
[0024] FIG. 5 is a perspective diagram that schematically shows the
filter of an exhaust gas purification device according to an
embodiment;
[0025] FIG. 6 is a diagram that schematically shows a portion of
the end face of a filter according to an embodiment;
[0026] FIG. 7 is a diagram that schematically shows a portion of
the end face of a filter according to an embodiment;
[0027] FIG. 8 is a graph that shows the relationship between the PM
accumulation time and the PM trapping ratio; and
[0028] FIG. 9 is a perspective diagram that schematically shows a
filter according to an embodiment.
DESCRIPTION OF EMBODIMENTS
[0029] Preferred embodiments of the present invention are described
below based on the figures. Matters required for the execution of
the present invention but not particularly described in this
Description (for example, general matters such as those related to
the disposition of particulate filters in automobiles) can be
understood as design matters for the individual skilled in the art
based on the conventional art in the pertinent field. The present
invention can be implemented based on the contents disclosed in
this Description and the common general technical knowledge in the
pertinent field.
[0030] First, the construction of an exhaust gas purification
device according to an embodiment of the present invention is
described with reference to FIG. 1. The herein disclosed exhaust
gas purification device 1 is disposed in the exhaust system of an
internal combustion engine. FIG. 1 is a diagram that schematically
shows an internal combustion engine 2 and the exhaust gas
purification device 1 disposed in the exhaust system of this
internal combustion engine 2.
[0031] A mixture containing oxygen and fuel gas is fed to the
internal combustion engine (engine) according to the present
embodiment. The internal combustion engine causes this mixture to
undergo combustion and converts the energy of combustion to
mechanical energy. When this occurs, the post-combustion mixture
becomes an exhaust gas and is discharged into the exhaust system.
The internal combustion engine 2 with the structure shown in FIG. 1
has an automotive gasoline engine as its main component, but an
engine other than a gasoline engine (for example, a diesel engine)
can also be used.
[0032] The exhaust system in this engine 2 is described as follows.
An exhaust manifold 3 is connected to an exhaust port (not shown)
through which the engine 2 communicates with the exhaust system.
The exhaust manifold 3 is connected to an exhaust pipe 4 through
which the exhaust gas flows. The exhaust passage of the present
embodiment is formed by the exhaust manifold 3 and the exhaust pipe
4.
[0033] The herein disclosed exhaust gas purification device 1 is
disposed in the exhaust system of the engine 2. This exhaust gas
purification device 1 is provided with a catalyst section 5, a
filter section 6, and an ECU 7 and purifies harmful components (for
example, carbon monoxide (CO), hydrocarbon (HC), and nitrogen
oxides (NO.sub.x)) present in the discharged exhaust gas and also
captures the particulate matter (PM) present in the exhaust
gas.
[0034] The ECU 7 is a unit that carries out control between the
engine 2 and the exhaust gas purification device 1 and contains as
constituent elements a digital computer and other electronic
devices that are the same as for common control devices.
Specifically, an input port is provided at the ECU 7 and is
electrically connected to sensors (for example, a pressure sensor
8) that are disposed at respective locations at the engine 2 and/or
the exhaust gas purification device 1. By doing this, the data
detected at the individual sensors is transmitted via the input
port to the ECU 7 as electrical signals. In addition, the ECU 7 is
also provided with an output port. Via this output port, the ECU 7
is connected to individual locations at the engine 2 and the
exhaust gas purification device 1 and controls the operation of the
individual members through the transmission of control signals.
[0035] The catalyst section 5 is constructed to have a purification
capacity for the three-way components (NOx, HC, CO) present in the
exhaust gas and is disposed in the exhaust pipe 4, which
communicates with the engine 2. As shown in FIG. 1, it is
specifically disposed on the downstream side of the exhaust pipe 4.
The type of catalyst section 5 is not particularly limited. For
example, the catalyst section 5 may be a catalyst in which a
precious metal, e.g., platinum (Pt), palladium (Pd), rhodium (Rd),
and so forth, is supported. A downstream catalyst section may
additionally be disposed in the exhaust pipe 4 downstream from the
filter section 6. The specific construction of this catalyst
section 5 is not a characteristic feature of the present invention,
and a detailed description here has therefore been omitted.
[0036] The filter section 6 is disposed downstream from the
catalyst section 5. The filter section 6 is provided with a
gasoline particulate filter (GPF) that can capture and remove the
particulate matter (referred to below simply as "PM") present in
the exhaust gas. The particulate filter according to the present
embodiment is described in detail in the following.
[0037] FIG. 2 is a perspective diagram of a particulate filter 100.
As shown in FIG. 2, this particulate filter 100 is provided with a
filter substrate 10 and, disposed in the interior of this filter
substrate 10, cells 22, 24 in a regular arrangement. The various
materials and shapes heretofore used in applications of this type
can be used for this filter substrate 10 constituting the herein
disclosed particulate filter. For example, a honeycomb substrate
provided with a honeycomb structure formed from a ceramic, e.g.,
cordierite, silicon carbide (SiC), and so forth, or an alloy (e.g.,
stainless steel and so forth) can be advantageously used. A
honeycomb substrate having a cylindrical shape for its outer shape
(present embodiment) is provided as an example. However, in place
of a cylindrical shape, an elliptical shape or polygonal tubular
shape may be used for the outer shape of the substrate as a
whole.
[0038] FIG. 3 is a schematic diagram of an enlargement of a portion
of the end face of the particulate filter 100 on the exhaust gas
inflow side. FIG. 4 is a diagram of the IV-IV cross section of FIG.
3. As shown in FIG. 3, the particulate filter 100 has a wall-flow
part 20 and a straight-flow part 30.
[0039] <The Wall-Flow Part>
[0040] As shown in FIG. 3 and FIG. 4, the wall-flow part 20 is a
location where adjacent cells 22, 24 are plugged at the end faces
opposite from one another at the two end faces of the filter
substrate 10, and has an inlet-side cell 22, an outlet-side cell
24, and a wall 26. In this embodiment, a plurality of inlet-side
cells 22 and a plurality of outlet-side cells 24 are disposed in
alternation in a grid form.
[0041] An inlet-side cell 22 is open only at the end on the exhaust
gas inflow side, while an outlet-side cell 24 resides adjacent to
an inlet-side cell 22 and is open only at the end on the exhaust
gas outflow side. In this embodiment, the end of the inlet-side
cell 22 on the exhaust gas outflow side is plugged by a plugging
part 22a, while the end of the outlet-side cell 24 on the exhaust
gas inflow side is plugged by a plugging part 24a. The inlet-side
cell 22 and the outlet-side cell 24 should be configured with an
appropriate shape and size based on a consideration of the flow
rate and components of the exhaust gas supplied to the filter 100.
For example, the inlet-side cell 22 and the outlet-side cell 24 may
have various geometric shapes, e.g., a quadrilateral shape such as
a square shape, parallelogram shape, rectangular shape, trapezoidal
shape, and so forth; a triangular shape; another polygonal shape
(for example, a hexagonal shape or octagonal shape); or a circular
shape. In this embodiment, the inlet-side cell 22 and the
outlet-side cell 24 are octagonal cells having an octagonal shape
for the cross section orthogonal to the axial direction of the
filter substrate 10. In addition, the inlet-side cell 22 and the
outlet-side cell 24 are respectively formed of cells having the
same size (cross-sectional area) in the aforementioned cross
section.
[0042] A wall 26 is formed between an adjacent inlet-side cell 22
and outlet-side cell 24. This wall 26 partitions the inlet-side
cell 22 from the outlet-side cell 24. The wall 26 is a porous
structure that permits the exhaust gas to pass through it. The
porosity of the wall 26 is not particularly limited, but is
suitably approximately 50% to 70% and is preferably 55% to 65%.
When the porosity of the wall 26 is too small, PM may then end up
slipping through; on the other hand, an excessively large porosity
for the wall 26 is disfavored because the mechanical strength of
the particulate filter 100 then assumes a declining trend. The
thickness of the wall 26 is not particularly limited, but is
preferably approximately 200 .mu.m to 800 .mu.m. Within this wall
thickness range, an inhibitory effect on pressure loss increases
can be obtained without a loss in the PM trapping efficiency.
[0043] <The Straight-Flow Part>
[0044] As shown in FIG. 3, the straight-flow part 30 is provided
with a through cell 32 and is a location where the through cell 32
is not plugged at either end face of the filter substrate 10. In
this embodiment, a plurality of through cells 32 are disposed along
the diagonal directions of the grid formed by the inlet-side cells
22 and the outlet-side cells 24 and between an inlet-side cell 22
and an inlet-side cell 22 adjacent thereto and between an
outlet-side cell 24 and an outlet-side cell adjacent 24
thereto.
[0045] The through cell 32 completely penetrates the filter 100
along its axial direction. In other words, unlike the inlet-side
cell 22 and the outlet-side cell 24 described above, the through
cell 32 is open at both its end on the exhaust gas inflow side and
its end on the exhaust gas outflow side. The through cell 32 may be
configured with an appropriate shape and size considering the flow
rate and components of the exhaust gas supplied to the filter 100.
For example, the through cell 32 may have various geometric shapes,
e.g., a quadrilateral shape such as a square shape, parallelogram
shape, rectangular shape, trapezoidal shape, and so forth; a
triangular shape; another polygonal shape (for example, a hexagonal
shape or octagonal shape); or a circular shape. The through cell 32
may have the same shape as or a different shape from the inlet-side
cell 22 and the outlet-side cell 24. In this embodiment, the
through cell 32 is a quadrilateral cell having a quadrilateral
shape for its cross section orthogonal to the axial direction of
the filter 100. In addition, the disposition in this embodiment is
such that one side of the quadrilateral shape of the through cell
32 in this cross section is parallel to and opposite from one side
of the octagonal shape of an inlet-side cell 22 and an outlet-side
cell 24.
[0046] To produce the above-described filter 100, for example, a
slurry may be prepared in which the main component is a ceramic
powder, e.g., of cordierite, silicon carbide (SiC), and so forth,
and this may be molded by, for example, extrusion molding, followed
by firing. During this, the end on the exhaust gas outflow side of
the inlet-side cell 22 may be plugged with a plugging part 22a and
the end on the exhaust gas inflow side of the outlet-side cell 24
may be plugged with a plugging part 24a. The porous wall 26 may be
formed by mixing a combustible material powder, e.g., a carbon
powder, starch, or resin powder, into the slurry and then burning
off the combustible material powder. The porosity of the wall 26
can be freely controlled at this point by changing the particle
diameter and amount of addition of the combustible material
powder.
[0047] As shown in FIG. 4, with this exhaust gas purification
device the exhaust gas flows in from the inlet-side cell 22
disposed in the wall-flow part 20 of the filter 100. The exhaust
gas that has flowed in through the inlet-side cell 22 passes
through the porous wall 26 and reaches the outlet-side cell 24. In
FIG. 4, the route by which the exhaust gas flowing in through the
inlet-side cell 22 passes through the wall 26 and reaches the
outlet-side cell 24 is shown by the arrows. Here, because the wall
26 has a porous structure, the PM is captured, during the passage
of the exhaust gas through this wall 26, at the surface of the wall
26 and within the pores in the interior of the wall 26. The exhaust
gas that has passed through the wall 26 and reached the outlet-side
cell 24 is then discharged from the filter through the openings on
the exhaust gas outflow side.
[0048] With this exhaust gas purification device, the exhaust gas
continuously flows in through the inlet-side cell 22 disposed in
the wall-flow part 20 of the filter 100. With the progress of PM
capture by the wall 26 of the wall-flow part 20 as described above,
PM accumulates at the surface of the wall 26 and within the pores
in the interior of the wall 26. In addition, the exhaust resistance
of the wall-flow part 20 increases as the PM accumulates at the
wall-flow part 20, and because of this the amount of exhaust gas
passing through the wall-flow part 20 declines and the exhaust gas
overflowing from the wall-flow part 20 flows into the straight-flow
part 30. Thus, as PM accumulation at the wall-flow part 20 advances
in this exhaust gas purification device, the amount of exhaust gas
flowing through the wall-flow part 20 declines and the exhaust gas
assumes a preferential flow into the straight-flow part 30.
[0049] In this case, while a large pressure loss occurs at the
PM-loaded wall-flow part 20, the pressure loss at the plug-free
straight-flow part 30 is kept low. Due to this, the increase in the
pressure loss can be kept small for the filter 100 as a whole. In
addition, even when the wall-flow part 20 has become completely
clogged, a small maximum value for the pressure loss can be
achieved due to the flow of the exhaust gas in the straight-flow
part 30. This then makes it possible to prevent adverse effects
such as a deterioration in fuel efficiency and engine problems. An
exhaust gas purification device 1 having an even higher level of
performance can therefore be provided.
[0050] In addition, a plurality of inlet-side cells 22 and a
plurality of outlet-side cells 24 are disposed in alternation in a
grid form in the wall-flow part 20 of this exhaust gas purification
device 1. Moreover, through cells 32 are disposed along the
diagonal directions of this grid and between an inlet-side cell 22
and an inlet-side cell 22 adjacent thereto and between an
outlet-side cell 24 and an outlet-side cell adjacent 24 thereto.
With this construction, the exhaust gas overflowing from the
wall-flow part 20 rapidly flows into the through cells 32, and as a
consequence pressure loss increases can be even more effectively
suppressed.
[0051] The filter 100 of this embodiment is described in detail in
the following. FIG. 5 is an external perspective diagram of the
filter 100. In the cross section orthogonal to the axial direction
of this filter 100, the cross-sectional areas of the inlet-side
cells and the outlet-side cells present in the outer peripheral
region B of this cross section are larger in the filter 100 than
the cross-sectional areas of the inlet-side cells and outlet-side
cells present in the central region A of this cross section.
[0052] For example, in the example shown in FIG. 5, the cross
section orthogonal to the axial direction of the filter 100 (the
direction of exhaust gas flow, i.e., the longitudinal direction of
the wall-flow part) is approximately circular. In this case, and
letting R be the radius of this cross section of the filter 100,
for example, the central region A may be defined as the region from
the center C of the cross section of the filter 100 to at least
1/2R (preferably 3/4R to 19/20R) of the radius R. The outer
peripheral region B may be defined as the region from the outer
edge D of the cross section of the filter 100 to at least 1/20R
(preferably 1/10 to 1/2R) of the radius R. In this embodiment, the
central region A is defined as the region from the center C of the
cross section of the filter 100 to 9/10R of the radius R. In
addition, the outer peripheral region B is defined as the region
from the outer edge D of the cross section of the filter 100 to
1/10R of the radius R.
[0053] FIG. 6 shows a portion of the end face of the exhaust gas
inflow side for the central region A of the filter 100. FIG. 7
shows a portion of the end face of the exhaust gas inflow side for
the outer peripheral region B of the filter 100. As shown in FIG.
6, the plurality of inlet-side cells 22A and outlet-side cells 24A
present in the central region A are formed of octagonal cells each
having the same size. In addition, as shown in FIG. 7, the
plurality of inlet-side cells 22B and outlet-side cells 24B present
in the outer peripheral region B are formed of octagonal cells each
having the same size.
[0054] As shown in FIG. 6 and FIG. 7, the cross-sectional area of
the inlet-side cells 22B and the outlet-side cells 24B present in
the outer peripheral region B is larger than the cross-sectional
area of the inlet-side cells 22A and the outlet-side cells 24A
present in the central region A. For example, the ratio (S1/S2)
between the cross-sectional area S1 of an inlet-side cell 22A and
an outlet-side cell 24A present in the central region A and the
cross-sectional area S2 of an inlet-side cell 22B and an
outlet-side cell 24B present in the outer peripheral region B is
suitably not more than approximately 19/20 and is preferably not
more than 14/15 and is particularly preferably not more than 9/10
(for example, not more than 27/32). The area ratio (S1/S2) for the
herein disclosed inlet-side cells 22A, 22B and outlet-side cells
24A, 24B preferably satisfies 1/2<(S1/S2)< 19/20, more
preferably satisfies 2/3.ltoreq.(S1/S2).ltoreq. 14/15, and
particularly preferably satisfies 3/4.ltoreq.(S1/S2).ltoreq. 9/10.
In addition, the cross-sectional area S2 of the inlet-side cells
22B and the outlet-side cells 24B present in the outer peripheral
region B is preferably at least 0.1 mm.sup.2 larger than the
cross-sectional area S1 of the inlet-side cells 22A and the
outlet-side cells 24A present in the central region A and is more
preferably at least 0.3 mm.sup.2 larger. The herein disclosed art
can be advantageously executed with an embodiment in which, for
example, the cross-sectional area S2 of the inlet-side cells 22B
and the outlet-side cells 24B present in the outer peripheral
region B is at least 0.5 mm.sup.2 larger than the cross-sectional
area S1 of the inlet-side cells 22A and the outlet-side cells 24A
present in the central region A. In this embodiment, the plurality
of inlet-side cells 22A and outlet-side cells 24A present in the
central region A each have a cross-sectional area S1 of about 2.7
mm.sup.2. The plurality of inlet-side cells 22B and outlet-side
cells 24B present in the outer peripheral region B, on the other
hand, each have a cross-sectional area S2 of about 3.2 mm.sup.2. In
this embodiment, the cross-sectional areas of the inlet-side cells
22B and the outlet-side cells 24B present in the outer peripheral
region B are thus uniformly larger than those of the inlet-side
cells 22A and outlet-side cells 24A present in the central region
A. As described below, a catalyst coating layer (not shown) may
additionally be provided on the inner wall surfaces of the
inlet-side cells 22A, 22B and the outlet-side cells 24A, 24B. When
any cell is provided with a catalyst coating layer, the
aforementioned area ratio (S1/S2) should be satisfied for the total
with the catalyst coating layer for the prescribed amount of the
coating.
[0055] Here, a large exhaust gas flow rate occurs in high engine
operating load regions (for example, when the air intake for a
2.0-L engine is 20 g/sec or more), and due to this there is then a
tendency for the exhaust gas to flow in the filter 100 as a whole.
On the other hand, a low exhaust gas flow rate occurs in low engine
operating load regions (for example, when the air intake for a
2.0-L engine is less than 20 g/sec), and due to this there is then
a tendency for the exhaust gas to be concentrated in the central
region A of the filter 100. As a consequence, a trend is set up
whereby PM accumulation progresses more in the central region A of
the filter 100 than in the outer peripheral region B. As PM
accumulation progresses in the central region A, the exhaust
resistance in the central region A increases and exhaust gas
overflowing from the central region A flows into the outer
peripheral region B. Thus, as PM accumulation progresses in the
central region A, the amount of exhaust gas flowing in the central
region A declines and the exhaust gas flows in the outer peripheral
region B.
[0056] In this embodiment, the cross-sectional areas of the
inlet-side cell 22B and outlet-side cell 24B present in the outer
peripheral region B are larger than the cross-sectional areas of
the inlet-side cells 22A and outlet-side cells 24A present in the
central region A. Due to this, even when PM accumulation in the
central region A reaches a limit and exhaust gas flows to the outer
peripheral region B, the PM can be captured in the wall-flow part
20B of the outer peripheral region B with its large trapping
capacity. Accordingly, pressure loss increases due to clogging can
be suppressed while maintaining the PM trapping efficiency. Thus,
the exhaust gas purification device 1 according to the present
embodiment has a high PM trapping efficiency and exhibits low
pressure losses and at the same time can exhibit a long-term
retention of its characteristics.
[0057] In this case, for the example of an average flow rate
condition of 15 to 30 m.sup.3/min and assigning 100% (volume) to
the total amount of exhaust gas passing through the central region
A, the configuration is preferably such that the amount of exhaust
gas passing through the straight-flow part 30A of the central
region A is 1% to 10% (more preferably 2% to 5% and particularly
preferably 3.+-.1%). Stated differently, for the state prior to PM
accumulation, the configuration is preferably such that 90% to 99%
(more preferably 95% to 98% and particularly preferably 97.+-.1%)
of the total amount of exhaust gas flowing in the central region A
flows in the wall-flow part 20A. The aforementioned effects can be
realized at even higher levels at within the indicated range for
the exhaust gas flow amount ratio.
[0058] In addition, for the example of an average flow rate
condition of 15 to 30 m.sup.3/min and assigning 100% to the total
amount of exhaust gas passing through the outer peripheral region
B, the configuration is preferably such that the amount of exhaust
gas passing through the straight-flow part 30B of the outer
peripheral region B is not more than 8% (more preferably 0.5% to 3%
and particularly preferably 1.+-.1%). Stated differently, for the
state prior to PM accumulation, the configuration is preferably
such that 92% to 100% (more preferably 97% to 99.5% and
particularly preferably 99.+-.1%) of the total amount of exhaust
gas flowing in the outer peripheral region B flows in the wall-flow
part 20B. When the exhaust gas flow amount ratio is within the
indicated range, pressure losses are more effectively reduced and
excellent filter characteristics are maintained.
[0059] As shown in FIG. 6, in this embodiment the plurality of
through cells 32A present in the central region A are formed as
quadrilateral cells each with the same size. In addition, as shown
in FIG. 7, the plurality of through cells 32B present in the outer
peripheral region B are formed as quadrilateral cells each with the
same size. Moreover, as shown in FIGS. 6 and 7, the cross-sectional
area of the through cells 32B present in the outer peripheral
region B are uniformly smaller than the cross-sectional area of the
through cells 32A present in the central region A. Devising such
sizes for the through cells 32A, 32B makes it possible to simply
and easily realize a construction in which the cross-sectional
areas of the inlet-side cell 22B and outlet-side cell 24B of the
outer peripheral region B are larger than the cross-sectional areas
of the inlet-side cell 22A and outlet-side cell 24A of the central
region A, thereby providing a secure and reliable suppression of
reductions in the PM trapping efficiency.
[0060] For example, the ratio (S3/S4) between the cross-sectional
area S3 of a through cell 32A present in the central region A and
the cross-sectional area S4 of a through cell 32B present in the
outer peripheral region B is suitably at least approximately 1.1
and is preferably at least 1.15 and is particularly preferably at
least 1.2. The area ratio (S3/S4) for the herein disclosed through
cells 32A and 32B preferably satisfies
1.1.ltoreq.(S3/S4).ltoreq.1.5, more preferably satisfies
1.15.ltoreq.(S3/S4).ltoreq.1.4, and particularly preferably
satisfies 1.2.ltoreq.(S3/S4).ltoreq.1.3. In addition, the
cross-sectional area S3 of a through cell 32A present in the
central region A is preferably at least 0.01 mm.sup.2 larger than
the cross-sectional area S4 of a through cell 32B present in the
outer peripheral region B and is more preferably at least 0.03
mm.sup.2 larger. The herein disclosed art can be advantageously
executed with an embodiment in which, for example, the
cross-sectional area S3 of the through cell 32A present in the
central region A is at least 0.05 mm.sup.2 larger than the
cross-sectional area S4 of the through cell 32B present in the
outer peripheral region B. In this embodiment, the plurality of
through cells 32A present in the central region A each have a
cross-sectional area S3 of about 0.3 mm.sup.2. The plurality of
through cells 32B present in the outer peripheral region B, on the
other hand, each have a cross-sectional area S4 of about 0.25
mm.sup.2.
[0061] In a herein disclosed preferred embodiment, in the central
region A the ratio (S1/S3) between the cross-sectional area S1 of
the inlet-side cell 22A and outlet-side cell 24A and the
cross-sectional area S3 of the through cell 32A is suitably at
least approximately 5, preferably at least 6, and particularly
preferably at least 9. This area ratio (S1/S3) for the inlet-side
cell 22A, the outlet-side cell 24A, and the through cell 32A
present in the central region A preferably satisfies
5.ltoreq.(S1/S3).ltoreq.20, more preferably satisfies
6.ltoreq.(S1/S3).ltoreq.15, and particularly preferably satisfies
9.ltoreq.(S1/S3).ltoreq.12. The cross-sectional area S1 of the
inlet-side cell 22A and outlet-side cell 24A present in the central
region A preferably is at least 1.5 mm.sup.2 larger and more
preferably is at least 2 mm.sup.2 larger than the cross-sectional
area S3 of the through cell 32A present in the central region A.
The herein disclosed art, for example, can be advantageously
executed using an embodiment in which the cross-sectional area S1
of the inlet-side cell 22A and the outlet-side cell 24A present in
the central region A is at least 2.4 mm.sup.2 larger than the
cross-sectional area S3 of the through cell 32A present in the
central region A.
[0062] In a herein disclosed preferred embodiment, in the outer
peripheral region B the ratio (S2/S4) between the cross-sectional
area S2 of the inlet-side cell 22B and outlet-side cell 24B and the
cross-sectional area S4 of the through cell 32B is suitably at
least approximately 6, preferably at least 10, and particularly
preferably at least 12. This area ratio (S2/S4) for the inlet-side
cell 22B, the outlet-side cell 24B, and the through cell 32B
present in the outer peripheral region B preferably satisfies
6.ltoreq.(S2/S4) 20, more preferably satisfies
10.ltoreq.(S2/S4).ltoreq.18, and particularly preferably satisfies
.ltoreq.12 (S2/S4).ltoreq.15. The cross-sectional area S2 of the
inlet-side cell 22B and outlet-side cell 24B present in the outer
peripheral region B preferably is at least 2 mm.sup.2 larger and
more preferably is at least 2.5 mm.sup.2 larger than the
cross-sectional area S4 of the through cell 32B present in the
outer peripheral region B. The herein disclosed art, for example,
can be advantageously executed using an embodiment in which the
cross-sectional area S2 of the inlet-side cell 22B and the
outlet-side cell 24B present in the outer peripheral region B is at
least 2.9 mm.sup.2 larger than the cross-sectional area S4 of the
through cell 32B present in the outer peripheral region B.
[0063] 3.2 mm.sup.2 was used in the aforementioned embodiment for
the cross-sectional area S2 of the inlet-side cell 22B and the
outlet-side cell 24B present in the outer peripheral region B. The
cross-sectional area S2 of the inlet-side cell 22B and the
outlet-side cell 24B present in the outer peripheral region B is
not limited to this. For example, the cross-sectional area S2 of
the inlet-side cell 22B and the outlet-side cell 24B present in the
outer peripheral region B can be set to approximately 9 mm.sup.2 or
less (for example, at least 3 mm.sup.2 and not more than 9
mm.sup.2).
[0064] In addition, 2.7 mm.sup.2 was used in the aforementioned
embodiment for the cross-sectional area S1 of the inlet-side cell
22A and the outlet-side cell 24A present in the central region A.
The cross-sectional area S1 of the inlet-side cell 22A and the
outlet-side cell 24A present in the central region A is not limited
to this and also should be smaller than the cross-sectional area S2
of the inlet-side cell 22B and outlet-side cell 24B present in the
outer peripheral region B. For example, the cross-sectional area S1
of the inlet-side cell 22A and the outlet-side cell 24A present in
the central region A can be set to approximately 8.6 mm.sup.2 or
less (for example, at least 2.8 mm.sup.2 and not more than 8.6
mm.sup.2).
[0065] Also, 0.25 mm.sup.2 was used in the aforementioned
embodiment for the cross-sectional area S4 of the through cell 32B
present in the outer peripheral region B. The cross-sectional area
S4 of the through cell 32B present in the outer peripheral region B
is not limited to this. For example, the cross-sectional area S4 of
the through cell 32B present in the outer peripheral region B can
be set to approximately 1.5 mm.sup.2 or less (for example, at least
0.1 mm.sup.2 and not more than 1.5 mm.sup.2).
[0066] Also, 0.3 mm.sup.2 was used in the aforementioned embodiment
for the cross-sectional area S3 of the through cell 32A present in
the central region A. The cross-sectional area S3 of the through
cell 32A present in the central region A is not limited to this and
also should be larger than the cross-sectional area S4 of the
through cell 32B present in the outer peripheral region B. For
example, the cross-sectional area S3 of the through cell 32A
present in the central region A can be set to approximately 1.8
mm.sup.2 or less (for example, at least 0.11 mm.sup.2 and not more
than 1.8 mm.sup.2).
[0067] An even better expression of the previously described
effects can be obtained at within the indicated ranges for the
cross-sectional areas S1 to S4 of the cells 22A, 24A, 22B, 24B,
32A, and 32B.
[0068] For the instant exhaust gas purification device, the present
inventors prepared a filter (Example) in which, as shown in FIGS. 5
to 7, the cross-sectional area of the inlet-side cell 22B and
outlet-side cell 24B of the outer peripheral region B was larger
than for the inlet-side cell 22A and outlet-side cell 24A of the
central region A, and also prepared a filter (Comparative Example)
in which the inlet-side cells 22A and 22B and the outlet-side cells
24A and 24B in the two regions were the same size; introduced
exhaust gas under the same conditions for each; and measured the PM
accumulation time (Hr) and the PM trapping ratio (%). Specifically,
in each example the exhaust gas purification device was placed in
the exhaust system of a gasoline engine and an exhaust gas
throughflow was established at a steady-state operation. A PM
sensor was placed upstream and downstream from the filter. The PM
trapping ratio (%) was measured while PM was accumulating in the
filter. Here, "(value measured by the PM sensor installed
downstream from the filter/value measured by the PM sensor
installed upstream from the filter).times.100" was used for the PM
trapping ratio. The results are shown in FIG. 8.
[0069] As shown in FIG. 8, the exhaust gas purification device
(Example) in which the cross-sectional area of the inlet-side cell
22B and outlet-side cell 24B of the outer peripheral region B was
larger than that of the inlet-side cell 22A and outlet-side cell
24A of the central region A, presented a greater suppression of the
decline in the PM trapping rate than did the exhaust gas
purification device (Comparative Example) in which the inlet-side
cells 22A and 22B and the outlet-side cells 24A and 24B in the two
regions were the same size. It was thus confirmed from these
results that the decline in the PM trapping ratio post-PM
accumulation is more favorably suppressed by making the
cross-sectional area of the inlet-side cell 22B and the outlet-side
cell 24B of the outer peripheral region B larger than that of the
inlet-side cell 22A and outlet-side cell 24A of the central region
A.
[0070] As shown in FIG. 3, a catalyst coating layer (not shown) may
additionally be provided in the wall-flow part 20 and the
straight-flow part 30. For example, in the wall-flow part 20, a
catalyst coating layer can be provided at the surface of the wall
26 and/or in the pores in the interior of the wall 26. A catalyst
coating layer formed on the inner wall surface of the through cell
32 can additionally be provided in the straight-flow part 30. In
these cases, the catalyst coating layer may contain a porous
support and a precious metal catalyst loaded on this support. Such
a construction makes it possible to carry out a favorable
purification of the harmful components (for example, carbon
monoxide (CO), hydrocarbon (HC), NO.sub.x, and so forth) in the
exhaust gas passing through the wall-flow part 20 and the
straight-flow part 30.
[0071] The support used for the catalyst coating layer can contain
one or two or more elements (typically as the oxide) selected from,
e.g., alkali metal elements (typically alkali metal oxides),
alkaline-earth metal elements (typically alkaline-earth metal
oxides), rare-earth elements (typically rare-earth oxides), Zr
(typically zirconia), Si (typically silica), Ti (typically
titania), and Al (typically alumina). The use of a support
containing these components can realize at least one (and
preferably all) of the following: an increase in the mechanical
strength, an improvement in durability (thermal stability), an
inhibition of catalyst sintering, and an inhibition of catalyst
poisoning. The alkaline-earth metal element can be exemplified by
magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The
rare-earth metal element can be exemplified by lanthanum (La),
scandium (Sc), yttrium (Y), cerium (Ce), praseodymium (Pr),
neodymium (Nd), samarium (Sm), and ytterbium (Yb). For example, one
or two or more oxides such as alumina (Al.sub.2O.sub.3), ceria
(CeO.sub.2), zirconia (ZrO.sub.2), titania (TiO.sub.2), silica
(SiO.sub.2), and so forth, are preferably used.
[0072] The precious metal catalyst used in the catalyst coating
layer can contain one or two or more elements selected from the
platinum group elements. The use of a precious metal catalyst
containing these components makes it possible to more reliably
purify the harmful components (for example, carbon monoxide (CO),
hydrocarbon (HC), NOx, and so forth) in the exhaust gas that passes
through the wall-flow part 20 and the straight-flow part 30. The
use of rhodium (Rh) in combination with palladium (Pd) or platinum
(Pt) is preferred. The use of Rh in combination with Pd or Pt makes
possible an even more efficient purification of the harmful
components in the exhaust gas. The amount of the precious metal
catalyst that is supported is not particularly limited, but
approximately 0.5 g to 20 g (preferably 1 g to 10 g) per 1 L of
filter volume is preferred. A satisfactory catalytic activity may
not be obtained when the amount of supported precious metal
catalyst is too small. When the amount of supported precious metal
catalyst is too large, the effect due to the loading of the
precious metal catalyst tapers off and higher costs are induced,
and is thus to be avoided.
[0073] The amount of formation of the catalyst coating layer is not
particularly limited, but, for example, is preferably approximately
5 g to 500 g (preferably 10 g to 200 g) per 1 L of filter volume.
When the amount of the catalyst coating layer per 1 L of filter
volume is too small, the functionality as a catalyst coating layer
is weak and there is also a risk of causing grain growth by the
supported precious metal catalyst. When the amount of the catalyst
coating layer is too large, this risks causing an increased
pressure loss when the exhaust gas passes through the wall-flow
part 20 and the straight-flow part 30.
[0074] With regard to the method of loading the catalyst coating
layer on the filter, for example, the filter substrate 10 may be
immersed in a slurry in which the catalyst components are
dispersed. A method can be used in which, after the slurry has been
impregnated into the filter substrate 10, drying and firing are
carried out in order to immobilize and support the catalyst
components at the wall 26 and/or at the inner wall of the through
cell 32.
[0075] The catalyst coating layer may be formed into a layered
structure having an upper layer and a lower layer wherein the lower
layer is closer to the surface of the filter substrate 10 and the
upper layer is relatively removed therefrom. In this case, for
example, Pd or Pt may be supported in one layer separately from Rh
in the other layer. By doing this, a suppressing effect can be
obtained on the reduction in catalytic activity that is caused by
the alloying of Rh with Pd or Pt. This may also be a layered
structure of three or more layers that has an additional layer or
layers besides the two layers.
[0076] An exhaust gas purification device 1 according to an
embodiment of the present invention has been described in the
preceding, but the exhaust gas purification device according to the
present invention is not limited to this embodiment. In the example
shown in FIGS. 5, 6, and 7, the cross-sectional area of the
inlet-side cell 22B and outlet-side cell 24B present in the outer
peripheral region B is uniformly larger than the cross-sectional
area of the inlet-side cell 22A and outlet-side cell 24B present in
the central region A. The cross-sectional areas of the inlet-side
cells 22A, 22B and outlet-side cells 24A, 24B formed in the filter
are not limited to this embodiment. For example, as in the filter
200 shown in FIG. 9, inlet-side cells and outlet-side cells may be
formed for which the cross-sectional area undergoes a gradual
(stepwise) increase moving from the center C of the cross section
to its outer edge D.
[0077] In the example shown in FIG. 9, the central region A is
defined as the region out to 1/2R of the radius R from the center C
of the cross section of the filter 200. The outer peripheral region
B is defined as the region to 1/5R of the radius R from the outer
edge D of the cross section of the filter 200. In addition, an
intermediate region E is defined as the region not included by the
outer peripheral region B and the central region A. In this case,
the inlet-side cells and outlet-side cells formed in the central
region A may have the smallest cross-sectional area. The inlet-side
cells and outlet-side cells formed in the intermediate region E may
have a cross-sectional area larger than that of the inlet-side
cells and outlet-side cells formed in the central region A.
Moreover, inlet-side cells and outlet-side cells having the largest
cross-sectional area may be formed in the outer peripheral region
B. Thus, in the filter 200 shown in FIG. 9, the inlet-side cells
and outlet-side cells are formed with cross-sectional areas that
undergo a stepwise increase moving from the center C of the cross
section to the outer edge D.
[0078] With the use of this filter 200, in the filter 200 the
amount of exhaust gas passing through the inlet-side cells and
outlet-side cells (wall-flow part) can be more finely tuned moving
from the center C of the cross section of the filter 200 to its
outer edge D. This in turn makes possible an even better
improvement in the PM trapping efficiency. Also in this case,
through a suitable selection of the sizes of the cross-sectional
areas of the through cells, inlet-side cells, and outlet-side cells
in the central region A, the intermediate region E, and the outer
peripheral region B, pressure loss increases can be suppressed
without impairing the PM trapping efficiency.
[0079] In the filter 200 shown in FIG. 9, the inlet-side cells and
outlet-side cells have been divided into three stages in terms of
the size of the cross-sectional area moving from the center C of
the cross section of the filter 200 to its outer edge D; however,
there is no limitation to this embodiment. For example, when a
plurality of rows of inlet-side cells and outlet-side cells are
formed from the center C of the filter cross section to its outer
edge D, the cross-sectional area may gradually increase with each
row moving from the center C of the cross section of the plurality
of inlet-side cell and outlet-side cell rows to the outer edge
D.
[0080] In another embodiment, the through cells (straight-flow
part) may be omitted from the central region A and the intermediate
region E of the filter. Thus, the unplugged through cells
(straight-flow part) may be provided only in the outer peripheral
region B of the filter. Again in this case, through a suitable
selection of the size of the cross-sectional area of the through
cells in the outer peripheral region B, pressure loss increases can
be suppressed without impairing the PM trapping efficiency.
[0081] Various examples of modifications of the exhaust gas
purification device 1 and particularly the particulate filter have
been provided above as examples, but the structure of the exhaust
gas purification device 1 and the particulate filter are not
limited to or by any of the embodiments described in the preceding.
In addition, the shape and structure of the individual members and
positions of the exhaust gas purification device 1 may also be
altered. This exhaust gas purification device 1 is, for example,
particularly suitable as a device that captures the PM present in
an exhaust gas having a relatively high exhaust temperature, such
as a gasoline engine. However, the exhaust gas purification device
1 according to the present invention is not limited to the
application of capturing the PM in the exhaust gas from a gasoline
engine and can be used in various applications for capturing the PM
in the exhaust gas discharged from other types of engines (for
example, diesel engines).
INDUSTRIAL APPLICABILITY
[0082] The present invention can provide an exhaust gas
purification device that can inhibit increases in the pressure loss
by the filter while maintaining PM trapping efficiency.
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